The present invention concerns a reactive diluent for a coating composition, and coating compositions including the same. The reactive diluent of the present invention comprises an hydroxyalkyl carbamate compound, as described in more detail below.
The reaction of propylene carbonate with primary and secondary amines to produce corresponding 2-hydroxypropyl carbamates is known in the art (Comp. rend, 1142, 1954). Similar reactions of ethylene carbonate are exemplified by the article, "The Preparation of Polymeric and Cyclic Urethans and Ureas from Ethylene Carbonate and Amines" by Elizabeth Dyer and Harvey Scott, J.A.C.S. (1956) pp. 672-675. See also the report "Polyurethane elastomers obtained without the use of diisocyanates" by L. Ya. Rappoport, G. N. Petrov, I. I. Trostyanskaya and O. P. Gavrilova in International Polymer Science and Technology, 8, No. 1, 1981. The Dyer-Scott reference discloses that polyurethanes might be prepared from 2-(hydroxyethyl) carbamate by elimination of ethylene glycol, thereby avoiding the need for using diisocyanates. The Rappoport et al paper discloses generally the reaction of cyclic carbonates with amines to form polyurethane elastomers. Thus, the prior art shows an awareness that amines react with, e.g., propylene carbonate, to yield the corresponding hydroxyalkyl carbamates. The Journal of Polymer Science, Vol. 7, 899 916 (1969), in an article entitled "New Method for Preparing Saturated and Unsaturated Aliphatic Polyurethanes" by Y. Mizake, S. Ozaki and Y. Hirata, at pages 899-915, discloses alternate routes to saturated and unsaturated polyurethanes, including polycondensation reaction of glycol bis (chloroformate) with diamine.
An article by Richard D. Cowell entitled: "Thermoplastic Polyurethane Elastomers: Chemistry Properties and Processing for the 80's" in the Journal of Elastomers and Plastics, Vol. 14, (October, 1982) pages 195-203, discloses the preparation of bis(2-hydroxyethyl)carbamates by reaction of diamines with ethylene carbonate followed by a catalyzed transesterification reaction with a glycol or macroglycol.
Coating compositions comprising a cross-linker and a backbone polymer containing sites thereon which are reactive with the cross-linker at elevated temperature to form a cross-linked polymeric material, but which are stable relative to each other at ambient temperatures are of course well known in the art. One difficulty with such compositions is that the high viscosity and high softening temperature of backbone polymers typically employed requires the utilization of a solvent to reduce viscosity of the polymer. After application of the coating composition and heating it to cure, volatilization of the solvent produces environmental, health and processing problems.
In order to achieve high solids content coating compositions, it is known to use high boiling point diols or polyols as reactive diluents in the compositions. However, in acid-catalyzed paint or coating formulations containing amino cross-linkers, the presence of diols or polyols reduces the shelf life of the coating formulation because the hydroxy groups on the polymer react with the amino cross-linkers.